Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view showing a first image of my new design; and,

FIG. 2 is a front view showing a second image of my new design.

The broken-lines in the drawings are for illustrative purposes only and forming no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 